1. Field of the invention
The present invention relates to a directional coupler using a strip line or a microstrip line, in particular using a microstrip line.
2. Description of the Prior Art
FIG. 9 shows a conventional branch-line directional coupler, in which a line 95' of a characteristic impedance Z.sub.1 is formed in a line 95 linking a terminal 91 and a terminal 93 over a length of a quarter-wavelength (1/4.lambda.s: .lambda.s is a wavelength on a transmission line of a ratio frequency signal), and a line 96' of the characteristic impedance Z.sub.1 is formed in a line 96 linking a terminal 92 and a terminal 94 over a length of a quarter-wavelength (1/4.lambda.s). On both sides of the quarter-wavelength (1/4.lambda.s) line 95' of the characteristic impedance Z.sub.1, one end of each of two one-quarter-wavelength lines 97 and 98 of a characteristic impedance Z.sub.2 are connected at an interval of a quarter-wavelength, and on both sides of the quarter-wavelength line 96' of the characteristic impedance Z.sub.1, the other end of each of the two one-quarter-wavelength lines 97 and 98 of the characteristic impedance Z.sub.2 are connected at an interval of a quarter-wavelength. The characteristic impedance Z.sub.1 of the one-quarter-wavelength lines 95' and 96' is selected to be lower than a characteristic impedance Z.sub.0 of the lines 95 and 96 (usually 50.OMEGA.), and the characteristic impedance Z.sub.2 of the one-quarter-wavelength lines 97 and 98 is selected to be higher than the characteristic impedance Z.sub.0 of the lines 95 and 96.
The radio frequency signal entering the terminal 91 may be designed to appear at the terminals 93 and 94, but not at the terminal 92. By adjusting the characteristic impedances Z.sub.1 and Z.sub.2, the coupling coefficient C between the terminal 91 and the terminal 94 (the ratio of the signal level delivered to the terminal 94 to the signal level entering the terminal 91) is determined.
In this branch-line directional coupler, the isolation between the terminal 91 and the terminal 92 (which is evaluated by the ratio of the signal level delivered to the terminal 92 to the signal level entering the terminal 91, and in which the smaller the ratio, the better the isolation) is excellent.
In this branch-line directional coupler, the coupling coefficient C between the terminal 91 and the terminal 94 was relatively large, and one of smaller coupling coefficient C could not be obtained, which was a disadvantage. For example, FIG. 10 is a characteristic diagram showing the relationship of the characteristic impedance Z.sub.1 of the one-quarter-wavelength lines 95' and 96', the characteristic impedance Z.sub.2 of the one-quarter-wavelength lines 97 and 98, and the coupling coefficient C between the terminal 91 and the terminal 94 at the center frequency, in the branch-line directional coupler in FIG. 9, and the relationship is as follows. ##EQU1## In this case, the coupling coefficient C and the coupling coefficient .beta. are satisfying the relationship of C=20log.beta..
When composing a branch-line directional coupler in a microstrip line on a dielectric substrate with a relative dielectric constant of 2.5 and a thickness of 0.6 mm, for example, a PTFE glass cloth substrate, if narrowing the line width W of two one-quarter-wavelength lines 97 and 98, a practical limit is 100 microns, and the characteristic impedance Z.sub.2 of the one-quarter-wavelength lines 97 and 98 at this time is about 160.OMEGA., and it was hence difficult to reduce the coupling coefficient C between the terminal 91 and terminal 94 markedly from -10 dB.
Thus, in the prior art described above, although the characteristic of excellent directivity may be easily obtained, it is hard to obtain the characteristic of the small coupling coefficient C of -10 dB or less.
Furthermore, in this branch-line directional coupler, the coupling coefficient C of the terminal 91 and terminal 94 is relatively large, and as approaching -3 dB, the characteristic impedance Z.sub.2 of the one-quarter-wavelength lines 97 and 98 which are branch lines comes to be closer to the characteristic impedance Z.sub.0, while the line width W of the one-quarter-wavelength lines 97 and 98 approaches the line width W.sub.0 of the characteristic impedance Z.sub.0. However, when composing a directional coupler in a microstrip line with a coupling coefficient C=-3 dB in a 14 GHz band on a dielectric substrate having a relative dielectric constant of 2.5 and a thickness of 0.6 mm, such as a PTFE glass cloth substrate, the characteristics are 1/4.lambda.s=3.5 mm. W=W.sub.0 =1.7 mm, and it is not clear from where to where is the actual length of the one-quarter-wavelength lines 95' and 96', or from where to where is the actual length of the one-quarter-wavelength lines 97 and 98, and designing of line patterns of the directional coupler was difficult, and the characteristics exactly as designed could not be obtained.